A plunger type master cylinder used for the purposes as mentioned above has a relief port formed in a piston capable of moving in an axial direction within a cylinder bore of a cylinder housing. It is said that a seal ring for opening and closing the relief port is high in durability.
As technical problems to be solved for such a plunger type master cylinder, firstly, there is such a problem as how to reduce the invalid stroke caused by braking operation and secondly, there is such a problem as how to prevent the residue of a brake fluid pressure (so-called residual pressure) in a fluid pressure chamber during non-operation of the brake. As a related art, there can be listed, for example, the Official Gazette of DE 19,536,325A1 (see especially its FIG. 1). In this related art, a technique for reducing the invalid stroke which belongs to the first-mentioned problem to be solved is disclosed, in which a piston (13) including a relief port (20) is provided at its tip part with a reduced-diameter part (40), and a seal ring (11) is arranged at the reduced-diameter part (40). As another related art, there can also be listed the Official Gazette of Japanese Patent Application Laid-Open No. 2000-108878 (see especially its FIG. 2). This related art also discloses a technique for reducing the first-mentioned invalid stroke, in which a piston (13) is provided with a control tapered-surface (86) which is formed on the outer peripheral surface of a piston (13) at the rear side of an opening part of a relief port (56). The inner periphery of a cup seal (46) can be sealed by utilizing this control tapered-surface (86). Moreover, Official Gazette of Japanese Patent Application Laid-Open No. 2001-146157 (see especially its FIG. 2) discloses a technique for preventing the second-mentioned residual pressure, in which a relief port (4c) is formed at a piston (4) and a safety hole (4e) is disposed at a rear side of the relief port, so that pressure in a fluid pressure chamber can positively be relieved through the safety hole.